1. Field of the Invention
This invention relates to focus detection devices for photographic lenses, zoom lenses or other optical instruments (hereinafter merely referred to as "objective lens"), and more particularly to focus detection devices which can provide an improved optical performance with increased accuracy of focus detection.
2. Description of the Prior Art
From the past, it has been known in the art to provide the so-called secondary image forming type in-focus detecting method in which, in rear of an image of an object to be photographed formed by an objective lens, there is arranged an additional lens for forming a second image of the object on the surface of photo-sensitive elements so that the photo-sensitive elements produce signals representing the degree of sharpness of the image formed thereon, the signals being used in detecting when the objective lens is in focus as, for example, disclosed in U.S. Pat. Nos. 3,511,156, 3,875,401 and 3,860,935.
FIGS. 1 and 2 are schematic views of optical systems for the conventional secondary imaging type in-focus detecting method.
In FIG. 1, an image 4 of an object 1 to be photographed is formed by an objective lens 1 near split prisms 3.sub.1 and 3.sub.2, divided into upper and lower or two parts by the split prisms 3.sub.1 and 3.sub.2 and then reformed by a second-image forming lens 5 on a pair of photo-sensors 6.sub.1 and 6.sub.2 as separate images 4.sub.1 and 4.sub.2 from each other. The photo-sensors 6.sub.1 and 6.sub.2 each consist of a plurality of elements 6.sub.11, 6.sub.12, . . . or 6.sub.21, 6.sub.22, . . . By using output signals from these photo-sensitive elements, the in-focus condition of the objective lens 2 is detected.
That is, when the objective lens 2 is in focus, the object image 4 after having been separated to the two images by the split prisms 3.sub.1 and 3.sub.2 is reformed as two images vertically exactly equivalent to each other on the surfaces of the photo-sensitive elements 6.sub.1 and 6.sub.2. When the objective lens 2 is out of focus, after having similarly been separated by the split prisms 3.sub.1 and 3.sub.2, it is reformed as two images of unsharp degree in horizontally sheared relation on the surfaces of the photo-sensitive elements 6.sub.1 and 6.sub.2.
Therefore, upon detection of the amount of shearing of these two images by the photo-sensitive elements 6.sub.1 and 6.sub.2, the out-of-focus condition can be detected. It is to be noted in this connection that whether or not the optical performance of the second-image-forming lens 5 is good contributes one factor on which the accuracy of detection of the in-focus condition depends.
FIG. 2 illustrates another method which is to use a pair of second-image forming lenses 7.sub.1 and 7.sub.2 arranged in rear of the prescribed focal plane F of the objective lens 2 symmetrically with respect to the optical axis of the lens 2 so that the image 4 of the object 1 is divided into two images by the second-image forming lenses 7.sub.1 and 7.sub.2 on respective photo-sensors 8.sub.1 and 8.sub.2 each consisting of a plurality of elements 8.sub.11, 8.sub.12, . . . or 8.sub.21, 8.sub.22, . . .
When the objective lens 2 is in focus, the two images on the surfaces of the photo-sensors 8.sub.1 and 8.sub.2 take their places in vertically equivalent positions to each other. When out of focus, the two images on the surfaces of the photo-sensors 8.sub.1 and 8.sub.2 are of unsharp degree in horizontally sheared relation to each other. By sensing the amount of shearing of the images, the in-focus condition is detected.
As in the above, the in-focus detection device of FIG. 2 makes use of a pair of second-image forming lenses 7.sub.1 and 7.sub.2 in rear of the prescribed focal plane F of the objective lens in combination with a pair of photo-sensitive element arrays in optically equivalent positions to the two images by this pair of second-image forming lenses 7.sub.1 and 7.sub.2, thereby the outputs from the two photo-sensitive element arrays are compared with each other to distinguish the degrees of sharpness and unsharpness from each other.
Since the object image by the objective lens 2 is reformed by the second-image forming lens near the pair of photo-sensitive element arrays in detecting the in-focus condition of the objective lens 2, the imaging performance of the second-image forming lens gives a large influence to the accuracy of detection of the in-focus condition.
Because it has been the prior art practice that the second-image forming lens is constructed from one positive lens, the second image on the photo-sensitive element is associated with large aberrations. Therefore, it cannot be said that the accuracy of detection of the in-focus condition is satisfactory.